The paper analyses the sources of the res judicata effect of international arbitral awards. It discusses the problems inherent in the application of the rules of domestic law governing the res judicata effects of national judgments and the approach of international arbitrators and of national courts. It then proposes the development of ad hoc transnational principles to govern the subject matter, and focuses in particular on the Recommendations on Res Judicata in International Commercial Arbitration of the International Law Association.